1. Field of Invention
The present invention relates to an active sonar tracking system providing cable location information for shallow water seismic data acquisition.
2. Description of the Prior Art
During seismic exploration of formations beneath relatively shallow bodies of water, acoustic ranges can be very short. Further, the transponders on the seismic cable and the transceiver on the seismic vessel must operate in an environment of high reverberation and multipath signals. Both phenomena are severely destructive of both acoustic signal amplitude and phase.
Large gradients in the speed of sound velocity profile, caused by water surface temperature and salinity, tend to drastically limit acoustic range. Shadow or dead zones are caused as the acoustic rays bend downwardly to the bottom of the body of water and then are reflected upwards toward the water surface. Acceptable positioning can, however, be achieved when range data is available on at least a fifty to sixty percent basis. For this reason, it is desirable to receive range signals from multiple transponder locations as the source vessel travels along its line of profile.